This invention relates to a hydraulic brake system for automotive vehicles with a master cylinder and with a hydraulic power booster connected upstream of the master cylinder and having a pressure chamber in which an auxiliary pressure depending on the actuating force may be modulated via a pedal-operable brake valve, with wheel brakes of a first brake circuit being connected at the pressure chamber and with wheel brakes of a second brake circuit being connected at the master cylinder, in which wheel brakes the braking pressure is additionally influenceable by valve means controllable in dependent on the slip, and in case of control a hydraulic communication being established between the pressure chamber of the hydraulic power booster and the master cylinder.
A device featuring the above characteristics is disclosed in the prior German patent application No. P 32 32 051.5. The brake system according to the prior suggestion substantially comprises a hydraulic power booster wherein two bores are arranged in parallel with each other. One of the cylinder bores accommodates the control slide of a brake valve. Via a lever actuation in dependence on the actuated force applied to the brake pedal, by means of said control slide it is possible to adapt and supply a hydraulic pressure to be metered into the pressure chamber of the hydraulic power booster. The pressure chamber of the hydraulic power booster is confined by a booster piston mechanically coupled with a master cylinder. Via valve means operable in dependence on the slip, a connection is established between the pressure chamber of the hydraulic power booster and a first brake circuit, thus the wheel brakes of this brake circuit being dynamically pressurizable. The master cylinder connected downstream of the hydraulic power booster is a tandem master cylinder with two working chambers pressurizable by the hydraulic power booster, on account of the interposition of the respective valve means operable in dependence on the slip each time a further brake circuit being pressurizable. The master cylinder further has a so-called prechamber. In dependence on the control signals of slip electronics, further valve means may connect said prechamber with the pressure chamber of the hydraulic power booster.
If in the described brake system an actuating force is exerted on the brake pedal there will be a corresponding displacement of the control slide of the brake valve. Thus, finally pressure medium of an auxiliary pressure source will flow into the pressure chamber of the hydraulic power booster. On the one hand, said pressure medium will effect a displacement of the booster piston in the direction of actuation. On the other hand, it will reach the wheel brakes of one brake circuit. Due to the displacement of the booster piston a hydraulic pressure will likewise build up in the working chambers of the master cylinder by means of which pressure the two other brake circuits will be pressurized. If slip control electronics monitoring the rotational behavior of the wheels detect unfavorable slip values at one or several vehicle wheel(s) the valve means inserted into the brake circuits will be switched over so as to reduce the braking pressure until a reacceleration of the respective wheel(s). At the same time, the prechamber of the tandem master cylinder will be pressurized. Thus, if required, the pressure medium tapped from the wheel brakes will also be replaced from the dynamic brake circuit.
From the above there results in the brake system according to the prior art a so-called Y-allotment of the brake circuits had been chosen in which, according to a certain selection procedure, the wheel brakes of the rear axle are jointly controlled via valve means operable in dependence on the slip while each one of the wheel brakes provided at the front axle is connected to one working chamber of the tandem master cylinder, thus an individual wheel control being possible at the front axle of the automotive vehicle.
In the prior art German patent application No. P32 47 498.9, on the other hand, it is disclosed that a first brake circuit is connected at the pressure chamber of a hydraulic power booster while a second brake circuit likewise comprising two wheel brakes is connected at the working chamber of a master cylinder. The arrangement of the wheel brakes at the vehicle theoretically may be effected in any way. For safety reasons and because of practical considerations, however, it has provide expedient to either arrange the wheel brakes of one brake circuit at one vehicle axle at a time or, on the other hand, to arrange the wheel brakes of each brake circuit at the vehicle so that they will diagonally oppose each other. If the wheel brakes of the brake circuits, e.g., are arranged at the vehicle so as to diagonally oppose each other a joint braking pressure control will not easily be possible in the wheel brakes of one vehicle axle. Thus, as a rule, there must be an individual wheel control of the automotive vehicle, thereby the number of the braking pressure control units increasing as compared with the brake system described before. If namely in case of a diagonal brake circuit allotment an individual wheel control of the automotive vehicle is to be realized it will be necessary to associate each wheel brake with corresponding valve means. As a rule, each valve means consists of an electromagnetically operable valve which is open when de-energized and which in the case of normal braking actions establishes a communication between the brake pressure source and the respective wheel brake. A further component of the valve means is a solenoid valve closed during normal braking actions and establishing a hydraulic communication between the wheel brake and an upressurized return reservoir upon a corresponding actuation by slip control electronics. In such a pressure reduction phase, the valve which is open when de-energized will be kept in a closed position. Thus, altogether, eight electromagnetically operable valves will be required if a diagonal brake circuit allotment is demanded in a brake system with slip control and if the pressure supply to one brake diagonal shall be dynamic and the pressure supply to the other diagonal shall be static.
It is thus an object of the present invention to reduce the number of the solenoid valves required in a hydraulic brake system in the presence of such demands.